If you send a letter from the US to London does it require more than one stamp?
A normal 1oz letter to London is $1.10. Just takes a couple of days usually.
Here is the link to the USPS. AS 13 oz letter will cost $11.40 to mail.http://ircalc.usps.com/MailServices.aspx?country=10143&m=1&p=0&o=13